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System choices for Passivhaus developments

Submitted by Passivhaus Trus... on Mon, 09/14/2015 - 15:06

Passivhaus does not dictate any particular construction system. Indeed, practically any construction system can be used and adapted to achieve Passivhaus, though each will have its own advantages and challenges for your particular project.

The important issue to consider is which construction system is best suited to achieve the stringent requirements of Passivhaus – such as airtightness, elimination of thermal bridging and appropriate U-values– given the particular building type you are designing, its function and the form you are developing.

As a general rule avoid mixing different systems within one building, as this introduces interfaces that will make achieving Passivhaus challenging and more expensive than it needs to be.

Construction systems can generally be classified as lightweight with less thermal mass (such as timber or steel frame) or as heavyweight with more thermal mass (such as masonry or concrete frame), although elements of thermal mass can be introduced to timber or steel.

The high levels of fabric performance and carefully controlled solar gain lead to internal environmental conditions remaining very stable. Passivhaus does not generally require either light or heavy thermal mass, and other considerations such as function, occupancy and climate will infl uence this choice.

An important factor in choice of system, is the likely procurement route, and the availability of an appropriate contractor. Thus if you are working on a domestic project and local builders are experienced in traditional masonry, choose masonry as you are more likely to achieve Passivhaus by working with the available skill base.

On larger projects there may be pressure to use steel frame due to its familiarity to large contractors and consequent cost advantages, so you may need to choose steel frame and adapt it to suit Passivhaus.

Similarly, key aspects of construction detailing should inform system choice. Details and junctions need to be designed with construction and assembly on site under realistic conditions (eg weather!) in mind. This is to avoid site compromises or later changes under pressures such as time, cost and materials availability during construction that can easily compromise design and cost.

Key Passivhaus issues with each of the main construction systems are summarised below:

Steel frame:

• Ensure steel structure does not pass through the thermal envelope at floors, roofs or for ancillary elements of the building.

• Achieving airtightness on the inner side of the wall will be challenging, if not impossible, due to the number of structural penetrations. Ideally locate the airtightness line within the wall and on the outside of the structure where it can be continuous and unbroken by the structure.

• Avoid heavy external cladding hung off the structure, or excessive cantilevered structures, as this will create unnecessary thermal bridging.

Timber frame:

• Consider the wall build up carefully, to avoid thermal bridging caused by solid studs, and use I-beams or a two layer wall with an inner structure and outer insulation wrap.

• Airtightness can generally be achieved with a membrane or racking board with tape over joints; racking boards are more robust. If a membrane is used on the internal face of the wall it is beneficial to have a service void to protect the membrane.

• Avoid elements of structure crossing the thermal envelope, for example, avoid roof overhangs with projecting joists that act as thermal bridges and make airtightness tricky.

Traditional masonry:

• This can create tricky detailing at window and door openings, and structural instability over large openings.

• It also creates voids under cavity barriers that are difficult to insulate on site.

• Airtightness can generally be achieved with parge or plaster.

• A wide cavity building on a concrete raft floating on insulation can be tricky, as the external skin requires support, so a thermal break will generally be required.

Concrete frame:

• Address potential thermal bridges at the connection of columns to foundations.

• Ensure concrete structure does not pass through the thermal envelope at floors, roofs or for ancillary elements of the building.

• Airtightness can be achieved with parge or plaster where external walls are infilled with masonry, or with membranes or board and tape, where infilled with lightweight construction such as timber or steel stud.

• Avoid heavy external cladding hung off the structure, or excessive cantilevered structures such as balconies, as this will create unnecessary thermal bridging.